Nested-flow heat exchangers and chemical reactors

ABSTRACT

Disclosed is a technology based upon the nesting of tubes to provide chemical reactors or chemical reactors with built in heat exchanger. As a chemical reactor, the technology provides the ability to manage the temperature within a process flow for improved performance, control the location of reactions for corrosion control, or implement multiple process steps within the same piece of equipment. As a chemical reactor with built in heat exchanger, the technology can provide large surface areas per unit volume and large heat transfer coefficients. The technology can recover the thermal energy from the product flow to heat the reactant flow to the reactant temperature, significantly reducing the energy needs for accomplishment of a process.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is continuation of U.S. patent application Ser. No.15/934,763, filed Mar. 23, 2018, entitled “NESTED-FLOW HEAT EXCHANGERSAND CHEMICAL REACTORS,” which is a divisional of U.S. patent applicationSer. No. 14/645,490, filed Mar. 12, 2015, (now U.S. Pat. No. 9,958,211,issued May 1, 2018), entitled “NESTED-FLOW HEAT EXCHANGERS AND CHEMICALREACTORS,” of which the entire contents of each is herein incorporatedin its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to equipment for processing chemicals andother fluid cycles bases on temperature change, including power cycles.The equipment includes heat exchangers and chemical reactors. Thepresent invention significantly reduces the capital investment requiredfor this type of equipment through use of standard constructiontechniques and materials. The present invention also significantimproved heat transfer performance, where needed, which significantlyreduces energy consumption for product production.

Description of the Prior Art

Heat exchangers and chemical reactors are commonly used equipment inchemical processing and recuperated power cycles. This equipment is usedin a broad spectrum of industries and in all stages of processing.

Heat exchangers are used to transfer heat (thermal energy) from oneprocess flow to another. Heat exchangers come in a variety of designs,including: tube-in-shell, flat plate, tube-in-tube, spiral-flow, andrecently developed bonded-etched-plates usually referred to as printedcircuit heat exchangers. Heat exchanger technologies are characterizedby their respective flow path hydraulic diameters and the heat transferarea per unit size. Both the hydraulic diameter and heat transfer areaper size scale inversely with the hydraulic diameter, thus the totalenergy transferred per unit time for a fixed temperature differencescales inversely with the square of the hydraulic diameter. This meansthat for a fixed sized heat exchanger, if one is constructed using tubesthat are one-half the tube size in the other, the one with the smallertubes can transfer four times the energy per unit time for the sametemperature difference. This has led to smaller and smaller hydraulicdiameter heat exchanger until now we are constructing heat exchangersusing printed circuit etching technologies. These heat exchangers usehydraulic diameters flow paths down to 0.5 mm and resulting areas pervolumes of up to 1300 m²/m³(http://www.heatric.com/typical_characteristics.html). But theconstruction technique for making printed circuit heat exchanger isquite complex and only a limited production capability exists, resultingin both high cost and long delivery times for these heat exchangers.Heat exchanger costs per performance determine the overall heatexchanger efficiency that one can afford to implement into a process. Ahigher cost heat exchanger technology limits the total heat exchangerefficiency due to the tradeoff between heat exchanger cost andreplacement energy cost. A lower cost heat exchanger technology achievesa higher efficiency as a result of that same tradeoff.

Chemical reactors are used to change a fluid from one chemical speciesto another. Chemical reactors operate at a variety of pressures andtemperatures, where these parameters are adjusted for best performancewithin the chemical reactors capabilities. Some chemical reactorsinclude catalyst while others have no need for catalyst, depending uponthe kinetics of the chemical reaction. Four examples of chemicalreactors are presented here for prior art description: hydrogenproduction using the Steam Methane Reforming (SMR) process; hydrogenproduction using the Water Shift Reaction (WSR) process; ammoniaproduction using the Haber-Bosch process; and urea production fromcarbon-dioxide and ammonia. These processes are used in commercialfertilizer production, which is one of the targets for application ofthis invention.

Today's SMR process involves the combustion of methane within acombustion chamber which is lined with pressurized and catalyst loadedreaction tubes in which the endothermic reaction of turning a mixture ofmethane and water to carbon-monoxide and hydrogen is accomplished.Although this process can be operated at low pressures, the tendency isto operate this process at 10-20 atmospheres to minimize the number ofreaction tubes and subsequent pumping operations. Because this processis an endothermic reaction, it is favored at higher temperatures andusually operated at temperatures between 850° C. and 1000° C. Thecatalyst is used to improve the reaction kinetics (speed of conversion)also in an effort to reduce the number of reaction tubes. The energyconversion in these systems is reported to be in the low 70% levels dueto several factors, these include: an additional 30% energy requirementto heat the SMR reactants from room temperature to reaction temperatureas compared to the energy needs to accomplish the process attemperature; and consumption of 35% of the combustion energy to raisethe combustion reactants from room temperature to SMR reactiontemperature. In addition to the low energy efficiency of today's SMRprocess, the capital cost of the system is high due to the strength ofmaterial required for the SMR reaction tubes. These tubes are operatedat high temperatures, while coupled to a low pressure source of heat;forcing their construction from expensive materials, addingsignificantly to the cost of the SMR system. Because the cost of naturalgas is significantly higher in Europe than it is in the US, severalEuropean fertilizer production facilities have incorporated a heatexchanger to recover the SMR product flow energy and utilize it forheating up the SMR reactant flow. This energy recuperation improves oneof the energy demands in this system.

Today's Water Shift Reaction (WSR) usually combines the carbon-monoxidefrom the SMR process and an outside source of steam to producecarbon-dioxide and hydrogen. Because of the characteristics of the WSR,the process is favored at lower temperatures, although a lowertemperature bond exists due to catalyst performance. The process isslightly exothermic, and in today's systems the WSR result in atemperature rise in the process flow temperature. This temperature riselimits the WSR conversion fraction, to offset that limitation the WSR isusually conducted in two stages with cooling provided between thesestages for higher conversion.

Today's Haber-Bosch process systems produce ammonia from hydrogen andnitrogen. The process is mildly exothermic, with a significant reductionin the number of molecules as a result of the chemical reaction. Thesecharacteristics result in more conversion at lower temperature as wellas higher pressure. The process is usually accomplished in two stages toavoid the temperature rise from a single stage and cooling is providedbetween the two stages. Even with these steps, the once throughconversion rate for the Haber-Bosch process is usually quite low,usually less than 20%. To obtain better utilization of the feed stock(hydrogen and nitrogen) the product flow is cooled to condense out theammonia, then reheated and returned to the inlet of the process toachieve large total conversion efficiencies. Today's system usesignificant energy to reheat the process flow and significant capitalinvestment to provide the pressure capability of the reactor and heatremoval capability of the increased flow through this system. At 18%once through conversion, the reactor size, the heat rejection system andthe system required to reheat the process flow to the desired operatingtemperature are twice the size of a system that can achieve 36% oncethrough conversion.

Today's urea production systems produce urea through the Bosch-Meiserprocess from a feedstock of ammonia and carbon-dioxide. The productionis usually described as a two-step process, with the ammonia andcarbon-dioxide first forming an ammonium-carbamate which includes oneionic bond, that ionic bond is replaced over time with a covalent bondand release of a water molecule. The ammonium-carbamate formation ismildly exothermic with a reduction in the number of molecules from threeto one. As a result, very high pressures and low operating temperaturesare desired to promote the product formation, with pressures in excessof 2000 psi commonly used. The ammonium-carbamate slowly converts tourea as an endothermic process with a 15-30 minute hold time usuallyused to achieve the urea conversion of approximately 80%. The energyneeds for the ammonium-carbamate to urea conversion is usually obtainedfrom the exothermic ammonium-carbamate formation. Although theammonium-carbamate can be removed from the urea through the simplereduction of the pressure over the mixture, today we accomplish thisremoval through the reduction of the partial pressure of either thecarbon-dioxide or ammonia over the mixture. This stripping processpermits the released reactants to be captured and returned as input feedto the process. The urea production reactor and the ammonium-carbamateto urea hold up column are very high pressure system. In addition, theammonium-carbamate is highly corrosive to metal because of its ionicnature. Because of this very high pressure and corrosive nature, thechemical reactor and transforming column are costly.

U.S. Pat. No. 7,645,437 describes an Integrated boiler, superheater, anddecomposer for sulfuric acid decomposition for the thermo-chemicalproduction of hydrogen. In that application we (Robert Moore, PaulPickard, Ed Pama, Fred Gelbard, Roger Lenard and Milton Vernon)described the nesting of ceramic tubes to accomplish the chemicalprocessing of sulfuric acid with a built in heat recovery design. Thatinvention permitted the highly concentrated sulfuric acid to enter theunit at approximately room temperature; be heated to nearly thedecomposition temperature through recovery of the sensible heat in theproduct discharge; be decomposed through the use of an external heatsource; and then be cooled by transferring the sensible heat of theproduct back to the sulfuric acid reactant feed. This permitted theconnections of the ceramic decomposer to be made with moderately lowtemperature materials, of which we had many choices. That patentdescribed only an endothermic process utilizing one flow path forreactants and one flow path for products.

U.S. Pat. No. 5,275,632 describes a bayonet reformer in which theoutside of a closed tube is heated with an externally fired source toheat the reforming reactant to produce hydrogen. This patent introducesthe concept of improved combustion thermal energy recovery through theuse of small gap annular combustion flow. This patent limits theimproved energy performance to the improved efficiency of thermal energyrecovery from combustion down to the operating temperature of thereformer, offers no thermal energy recovery of the combustion energybelow the reformer temperature and provides no recovery of the reformingproduct sensible heat.

U.S. Pat. No. 5,429,809 describes a bayonet chemical reactor in whichthe outside of a closed tube is heated with a thermal fluid that raisesthe process reactants to the desired temperature then that fluid limitsthe product temperature by acting as a heat source or sink as isnecessary. Although this invention limits the temperature swings withina process, it does nothing for improvement of efficiency.

U.S. Pat. No. 5,639,431 describes a bayonet reformer in which theemphasis of the patent is on removing hydrogen through the use ofhydrogen permeating tubes or cylinders, in an effort to lower theoperating temperature of the reformer and thus achieve some energyefficiency. This patent introduces a complicated membrane in an attemptto achieve added efficiency, but fails to address major efficiencylosses such as SMR product temperature and combustion exhausttemperature.

U.S. Pat. No. 5,876,469 describes a bayonet reformer in which theemphasis of the patent is on controlling the uniformity of the heatingto the pressure tubes of the reformer, which are in the bayonet design,and the recovery of the flue gas sensible heat down to the operatingtemperature of the reforming process. This invention fails to recoverflue gas sensible heat at temperatures less than the reforming processtemperature or recover any of the reforming sensible heat.

Against this background, the present invention was developed.

BRIEF SUMMARY OF THE INVENTION

Nested-flow technology is a system of flow channels developed throughthe nesting of circular tubes. These tubes provide a flow path boundedby the inside of an outer tube and the outside of an inner tube. Theflow path is maintained open through the use of spacers within the flowpath that do not substantially restrict flow. The spacing between theouter and inner tubes provides a characteristic hydraulic diameter andflow area; while the length of the tube provides a characteristic flowlength.

A single nested-flow unit refers to the aggregate of all tubes nestedwithin one another which essentially share the same axial centerline.The minimum number of nested tubes within a single nested-flow unit isdetermined from the number of process flows required to meet the desiredresult, while the maximum number of nested tubes is usually determinedfrom other considerations such as internal pressure and material stresslimits. The hydraulic diameter of flow channels can be the same ordrastically different depending upon the needs of the system.

A manifold nested-flow unit refers to the assembly of multiple singlenested-flow units into a common unit through the use of flowdistribution manifolds. Each of the single nested-flow units performsthe same process and it is the sum of these parallel operations thatdetermine the quantitative production capability of the manifoldnested-flow unit.

The nested-flow technology can be assembled into a heat exchanger by therepeated nesting of tubes one within the other, with each tube attachedto a separate manifold plate at each end of the tube. A simple trade-offcan be made between adding more tubes per single nested-flow unit andadding more single nested-flow units per production unit to minimize thecost of the system. The centermost tube of the single nested-flow unitcan provide a flow path on the inside of the tube or not; if it providesa flow path then a final manifold plate is included to provide acorresponding flow path in the manifold assembly; if the centermost tubedoes not provide a flow path, the tube could be replaced by a rod andthe rod or tube connected to the last manifold plate at both ends of thenested-flow tubes. For heat transfer between two fluid flows, with onefluid being heated while the other is being cooled, one flow path optionis to flow the fluid being heated in one direction within all oddnumbered channels, when channels are numbered from the centermostchannel, while flowing the fluid being cooled in the opposite directionwithin all even numbered channels. For heat transfer between three fluidflows, for example say two fluids being heated and one fluid beingcooled, one option is for the central most channel to be used for thefirst fluids being heated, the next channel out would be of the fluidbeing cooled and the third channel out would be of the remaining fluidbeing heated. This pattern would continue until all the flow channels inthe single nested-flow are consumed.

The Nested-flow Technology can be assembled into a chemical reactor bythe incorporation of chemical reactions sites within one or more ofthese nested-flow channels. Those chemical reaction sites are defined bythe presence of catalyst or by the mixing of two or more fluid flowsinto a common volume providing an initial start point for the chemicalreaction. In a single nested-flow unit the flow paths can be from oneend to the other or operated as a bayonet flow with the flow returningback to the end from which it originated. These flow paths can beutilized to transfer heat, provide isolation flows for corrosioncompatibility or provide resonance time for fluid flow to achieve thedesired chemical conversion. A phase change jacket can be incorporatedinto the chemical reactor as a method of adding or removing energy at aconstant temperature. Fluid can be added to the jacket at the desiredtemperature and state (vapor or liquid), the fluid can then change state(condense or boil) to either add energy to or remove energy from aprocess at a fixed temperature, then the fluid removed from the jacketin the new state. The fluid may be added or removed through atemperature gradient if the combined sensible and latent heat managementis desired. A simple boiler is one example of the use of a phase changejacket which could incorporate a temperature gradient heat-up flow.

It is a primary objective of the present invention to provide a heatexchanger which is more cost effective per unit of energy transferred atthe same driving temperature than we have today.

It is another objective of the present invention to provide a chemicalreactor that incorporates heat transfer between the reactant and theproducts at a sufficiently low cost that the reactor will provide asignificantly improved energy efficiency for the process.

It is another objective of the present invention to provide a chemicalreactor that provides heat transfer during chemical reactions to bettermanage the process for improved production per product pass andreduction in non-desirable product production.

It is another objective of the present invention to provide a chemicalreactor that utilizes the flow paths to limit the corrosion impact ofproduct produced.

These and other objectives of the present invention will become apparentto those skilled in this art upon reading the accompanying description,drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view of one set of nested tubes showing tubes andresulting flow channels for one design of a nested-flow heat exchanger.

FIG. 2 is a side view of one set of nested tubes with inner tubes pulledslightly out to reveal spacers used (in this case wire wrap) to maintainflow gap spacing.

FIG. 3 is a cutaway view of a manifold nested-flow unit where sevensingle nested-flow units, each consisting of eight tubes and a centralrod, are assembled onto one manifold.

FIG. 4 is a cutaway view of a single nested-flow unit used for ammoniaproduction.

FIG. 5 is a cutaway view of a single nested-flow unit used for SteamMethane Reforming.

FIG. 6 is a cutaway view of a single nested-flow unit used for WaterShift Reaction.

FIG. 7 is a cutaway view of a single nested-flow used for ureaproduction.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

Nested-flow technology as used herein shall mean a system of nestedtubes with associated flow channels, created by those tubes, which aremaintained as open flow channels through the use of spacers within thoseflow channels.

Single nested-flow unit as used herein shall mean a group of nestedtubes with a common axial centerline, more or less, providing as manyflow channels as desired.

Manifold nested-flow unit as used herein shall mean more than one singlenested-flow unit assembled onto a common manifold for achieving desiredproduct flow rates.

Wire wrap as used herein shall mean the installation of a small rod orwire spiraling from one end of a tube to the other end. The wire wrapcan spiral fairly quickly from one end of the tube to the other, meaningthat several inches of tube length would be incorporated for each wraparound the tube. The wire in this wire wrap need not be metal, but shallbe compatible to the tube onto which it is wrapped and the fluid flowingwithin the channel in which it resides.

Phase change jacket is a larger volume enclosed by a single tube whichcontains a number of single nested-flow units in which the principleheat transfer mechanism is either boiling or condensation.

Endothermic process as used herein shall mean a process in which energymust be added else the reaction temperature would decrease.

Exothermic process as used herein shall mean a process in which energymust be removed else the reaction temperature would increase.

Best Mode of the Invention

Best mode of the invention as contemplated by the inventor is tomaximize the energy savings within the design until the incrementalenergy savings from a size increase does not recover the added capitaland pumping cost. As contemplated by the inventor, any tube placedwithin another should first have a spacer installed on the outside ofthe tube, and in the inventors opinion that spacer should be a “wire”spiral wrap from, essentially, one end of the tube to the other. Thatwire wrap should be the gap size minus any uncertainty in that gap sizeas a result of size uncertainties in the tubes.

How to Make the Invention

Although a single nested-flow unit can be operated, most productionunits will consist of a multitude of single nested-flow units connectedto a common manifold system consisting of several plates, depending uponthe number of flow channels in the single nested-flow unit. A plate ismachined to accept the number of same size tubes needed to meet theusers desired flow as well as to provide a flow path for the fluid. Allnested-flow tubes of the same size are installed on the same manifoldplate, and every tube size within a single nested-flow unit has anindependent plate. Thus if seven size tubes are incorporated into thesingle nested-flow unit, then at least seven manifold plates areprovide, one for each tube size. If the inner-most tube is used for aflow channel, then an eighth plate is added to provide a correspondingchannel in the manifold. All tubes which go inside another are firstwire wrapped to provide a physical spacer to maintain the resulting flowpath open, then the tubes are attached to the manifold plates; thennested together with inclusion of catalyst, when needed, to produce amanifold nested-flow unit. The manifold can be welded, brazed, orincorporate gasket and seals to prevent loss of process fluid.

FIG. 1 shows the end view of a single set of nested tubes that can beused for a single nested-flow unit. In this arrangement, a center ⅛ inchwire wrapped rod 1 is slid into a 0.25 inch diameter 0.049 inch wallthickness wire wrapped tube 2 then six 0.035 wall thickness wire wrappedtubes each 0.125 inches larger than the other starting with a 0.375 inchtube 2 until the final 0.035 inch wall thickness 1.0 inch diameter wirewrapped tube 3. A final 1.125 inch diameter 0.049 inch wall thicknesstube 4 without wire wrap is then slid over this bundle of nested tubesto provide an eight flow channel single nested-flow unit. Thisarrangement provides six 0.028 inch flow gaps 7, 8, 9, 10 and 11 and two0.014 inch flow gaps 6 and 12. This nested-flow arrangement on ahexagonal configuration provides an area per volume of 850 m²/m³,rivaling the printed circuit heat exchanger area per volume at asignificantly less complicated construction effort.

FIG. 2 shows the side view of a single set of tubes that can be used fora single nested-flow unit. Within the outer most tube 13 are nestedtubes 14, 15, 16, and 17 along with a nested rod 18 each wire wrapped19, 20, 21, 22 and 23 in order to maintain open, for fluid flow, theflow path created by the tube arrangement. While the diameters of thesetubes are usually less than a few inches, the length of these tubes areseveral to twenty feet making the flow paths very thin and very long.These long flow paths permit the flow to reach turbulent behavior withhigh heat transfer effectiveness. Once in turbulent flow, the heattransfer effectiveness is nearly independent of flow rate, with only therising pressure drop due to increasing flow that limits the performance.

FIG. 3 shows a cutaway view of a manifold nested-flow unit where eachsingle nested-flow unit consists of eight nested tubes and a centralrod. Seven single nested-flow units (one in the center 24 and six aroundthat one 25, 26, 27, 28, 29 and 30) are assembled onto one manifold 31.In the nested-flow arrangement the outer tube of the nested-flow nestedtubes is attached to the upper plate, the next tube in is attached tothe next lower plate and so forth, until finally the rod located in thenested-flow's center is attached to the lowest plate. This manifoldarrangement permits the construction of large number of singlenested-flow units into a single production unit. It is only the abilityto handle this manifold unit size that limits its size.

How to Use the Invention

The nested-flow technology can be used as a heat exchanger or as achemical reactor or as a combined unit accomplishing both chemicalreaction and heat exchange.

FIG. 3 shows the basic configuration of the nested-flow technologyattached to one manifold using seven single nested-flows units, eachconsisting of one rod and eight tubes. This arrangement provides eightflow paths. When used as a two flow heat exchanger a similar manifold isinstalled at the other end of the tubes and every other flow path isused for one fluid flow and the remaining paths serve the remainingfluid flow.

EXAMPLES OF THE INVENTION

FIG. 4 shows a cutaway view of a single nested-flow unit used forammonia production. Reference centerline 54 is shown as a reminder thatthe configuration is a set of nested tubes where the created flow pathsare maintained through the use of a widely spaced spiral “wire” wraps.Ammonia is produced when the nitrogen and hydrogen reactants are heatedto an optimum temperature and exposed to a formation catalyst. Thatoptimum temperature is a tradeoff driven by the speed of formation whichfavors higher temperatures and extent of conversion favored by lowertemperatures. In the nested-flow unit, the energy to raise the reactantflow to the optimum temperature is obtained from the product flow.Innermost tube 32 is used to reduce the gap in the reactant channel 48created by the outside surface of tube 32 and the inside surface of tube33 to improve this heat transfer. Tube 32 is a relatively small diametertube, approximately 0.5 inches +/−0.25 inches in diameter but extendsmany feet into the nested-flow where it is sealed with an end plug. Thelength of tube 32 is driven by the need for heat transfer area. Cavity47 provides a volume for thermal couple insertion for chemical reactorcontrol if so desired. Tube 32 is attached to manifold plate 39 andmanifold plate 39 may have many similar tubes attached, one for eachsingle nested-flow unit in the final manifold nested-flow unit, allhaving a common reactant input 42. Tube 33 is used as a flow baffle toforce the flow from the cooler end of this unit to the warmer end andthen back to the cooler end. Tube 33 becomes the outer boundary of thereactant channel and the inner boundary of the product channel 49. Tube33 is attached to manifold plate 38, which may have many similar tubesattached, one for each single nested-flow unit in the final manifoldnested-flow unit, all having a common product outlet 41. Tube 33 isapproximately 0.625 inches +/−0.25 inches in diameter between themanifold connection and past tube 32, where its diameter is reduced topermit the placement of an ammonia production catalyst 44. Tube 33 isnot closed at the upper end, but left open permitting the reactants toflow from the inside of tube 33 to the outside at the warmer end of thenested-flow system. Tube 33 provides the heat transfer surface area forthe for the thermal energy exchange between the reactants in channel 48and the products in channel 49. Tube 34 provides the outer surface ofthe product flow channel. Tube 34 is attached to manifold plate 37,which may have many similar tubes attached. Tube 34 is approximately0.75 inches +/−0.25 inches and ten to twenty feet long. Tube 34 isclosed at the warm end of the nested-flow, forcing the reactants to flowacross the catalyst and form ammonia which is then cooled as it flowstowards the cooler end of the nested-flow by both the reactant flow 48and the water jacket flow 50. Tube 35 provides an outer boundary for thecooling jacket water flow. Tube 35 is attached to manifold plate 36,which may have many similar tubes attached. Manifold plate 36 providesfor a common water input 40 for all of the single nested-flow unitsinstalled in the manifold nested-flow unit. Tube 35 is approximately0.875 inches +/−0.25 inches in diameter, and runs from manifold plate 36to an intermediate manifold plate 51 just before the catalyst region ofthis nested-flow system. Manifold plate 51 is a transitional manifoldplate which goes from one tube around each nested-flow to a common tube52 around all nested-flow units attached to manifold 36. This commontube 52 provides a boiling region 45 for all single nested-flow unitsthat allow the system to be operated either vertically or horizontally.Tube 52 is approximately 2 ft+/−1 ft in diameter and 2 ft+/−1 ft inlength. A pressure vessel end cap 53 is attached to tube 52, with anexit port 46 near the top of the unit, which depends on the orientation,for steam to be used elsewhere in the production operation. This steamremoves nearly all of the heat of formation for the ammonia operation. Amanifold nested-flow unit for the production of twenty metric tons perday at a per cycle conversion fraction of 17% would be approximately onefoot in diameter and ten feet long. The nested-flow design for ammoniaproduction converts an ammonia production process from an energyconsumption process costing $12/ton-of-ammonia to an energy productionprocess worth $12/ton-of-ammonia based on natural gas cost of$4/million-BTUs.

Operation of the ammonia manifold nested-flow unit is accomplished byfirst heating the water in region 45 to the operation temperature forthe water. The reactant mixture of approximately one molar part nitrogenand three molar parts hydrogen is introduced at the common port 42. Thereactants are heated by the tube surfaces as they flow toward the warmend of the nested-flow, where they form ammonia when in contact with thecatalyst 44. Most of the heat of formation is deposited in the commonboiling water volume 45, where the steam produced is removed for useelsewhere. The ammonia and un-reacted hydrogen and nitrogen are cooledas they move towards the cooler end of the nested-flow, providing theheat to the tubes which is used to heat both the reactants and water totheir desired temperature. The flow from all the nested-flows isgathered inside the manifold where it exits at the common port 41. Theammonia will condense as it is cooled in flow channel 49. If operatedvertically, then the ammonia, nitrogen and hydrogen are separatedshortly after exiting the nested-flow manifold. If operatedhorizontally, then a lower and upper port can be incorporated for port41, permitting separation of ammonia from un-reacted reactants withinthe manifold. Significantly higher once through production rates willoccur because the temperature is not permitted to rise in the productionof ammonia. As a result, significantly less recycling will occur and thetotal ammonia production size will be smaller.

FIG. 5 shows a cutaway view of a single nested-flow used for SteamMethane Reforming (SMR). Centerline 73 is added to remind the readerthat the nested-flow consists of a series of nested tubes with amore-or-less common centerline. Steam Methane Reforming is a processwhere steam and methane are reacted at relatively high temperatures toproduce carbon-monoxide and hydrogen. The center most tube 55 is used toreduce the gap of the methane used as a combustion heat source thatdrives the endothermic SMR process and provides a cavity 61 for theinsertion of one or more thermocouples for process controls if sodesired. Wire wrap tube 55 has a diameter of approximately 0.5 inches+/−0.25 inches and is 8 feet to 18 feet long. Wire wrap tube 56 has adiameter of approximately 0.625 inches +/−0.25 inches, and provides theouter surface of the combustion methane flow channel 62 and the innersurface of the combustion air flow channel 63. The combustion reactantsare maintained in separate channels due to the tendency to auto-igniteprior to arriving at the desire combustion location 72. Wire wrap tube57 has a diameter of approximately 0.75 inches +/−0.25 inches, andprovides the outer surface of the combustion air channel and the innersurface of the combustion exhaust channel 64. Wire wrap tube 58 has adiameter of 0.875 inches +/−0.25 inches and provides the outer surfaceof the combustion exhaust channel and the inner surface of the SMRreactant channel 65. Tube 58 is plugged at the warm end of the assemblyto force the air and methane of the combustion process to start flowingdown the tube where the combustion process occurs. Wire wrap tube 59 hasa diameter of 1.0 inches +/−0.25 inches and provides the outer surfaceof the SMR reactant flow channel and the inner channel of the SMRproduct flow channel 66. Tube 60 has a diameter of 1.125 inches +/−0.25inches and provides the outer surface of the SMR products. Tube 60 isplugged at the warm end of the tube and is the outer tube of thenested-flow tubes. All of the single nested-flows units that areassembled for a manifold nested-flow unit are housed within a commonpressure tube 67. Tube 67 has a thermal insulator blanket 68 thatisolates tube 67 form the internal temperatures of the nested-flowunits, allowing the use of stainless steel throughout the system byminimizing the pressure differential across all tubes with the exceptionof tube 67.

The methane flow for combustion is through channel 62. This methane flowis heated as it moves toward the warm end of the nested-flow nestedtubes. The air flow for combustion is through channel 63. This air flowis heated as it moves toward the warm end of the nested-flow nestedtubes. The methane and air are mixed at the warm end of the nested-flowunit, where the temperatures are greater than 800° C. Since theauto-ignition temperature for methane air is approximately 550° C., themethane and air will auto-ignite. The reactant flow for the SMR processis a mixture of steam and methane introduced through channel 65. The SMRreactant flow is heated as it travels toward the warm end of thenested-flow nested tubes, but will not substantially react until itenters the SMR catalyst 70 region of the channel 71. The SMR process hasa significant entropy gain, going from three molecules of reactants tofour molecules of products and proceeds nearly to completion providedsufficient energy is provided through the combustion process occurringin region 72. By having the combustion process so tightly coupled withthe SMR process, the combustion process is only a few degrees hotterthan the SMR process. As a result, the methane combustion process of thenested-flow system will have an equilibrium level significantly higherthan 85% combustion level of the standard 2000° C. methane combustion.In addition, a combustion catalyst 69 is included in the system to takethe methane combustion to even higher values as the combustion gasescool from loss of energy to the SMR process. Assembled into a manifoldnested-flow unit, the SMR production unit will be twenty feet long andapproximately twenty inches in diameter to meet the hydrogen productionneeds for twenty tons of ammonia per day. The efficiency improvementprovided by the nested-flow technology will reduce the cost of hydrogenproduction from $140/ton-of-ammonia to $110/ton-of-ammonia based onnatural gas cost of $4/million-BTUs.

FIG. 6 shows a cutaway view of a single nested-flow used for Water ShiftReaction (WSR). Centerline 74 is added to remind the reader that thenested-flow consists of a series of nested tubes with a more-or-lesscommon centerline. The WSR is a mildly exothermic process in which onemole of carbon-monoxide and one mole of water is reacted to produce onemole of carbon-dioxide and one mole of hydrogen. The optimum reactiontemperature is a tradeoff driven by the speed of formation which favorshigher temperatures and extent of conversion favored by lowertemperatures. This WSR unit also serves to preheat the reactants for theSMR process and cool the products from the SMR to a lower temperature.This recuperation heat exchanger is built into the nested-tubearrangement of the WSR to same space. One option for the WSR unit is toincorporate it into the SMR unit producing a combined SMR/WSR unit. Forclarity, these two units are shown separately. Wire wrapped tube 75serves to improve the heat transfer between flows by decreasing the gapsin outer nested-flows. Wire wrapped tube 76 serves as a separationbarrier between the combustion methane flow 83 and combustion air flow84. Wire wrapped tube 77 serves as a separation barrier between thecombustion air flow and the combustion exhaust product flow 85. It isthe surface area of this tube and the hydraulic diameters of theadjacent flow channels that determine the temperature difference betweenthese two flows. Wire wrap tube 78 serves as the barrier between thecombustion exhaust flow and the WSR/SMR reactant flow 86. It is the heattransfer area of this tube and the hydraulic diameter of channels 86 and87 that are the hardware drivers for determining the temperaturedifference between these two flows. Wire wrapped tube 79 serves as thebarrier between the WSR/SMR reactant flow 86 and the WSR/SMR productflow 87. Wire wrapped tube 80 serves as the barrier between the WSR/SMRproduct flow 87 and the water cooling jacket flow 86 which removes theexcess heat from the WSR process. The SMR products and excess water flowdown channel 87 where they react when in contact with catalyst 90 inregion 91. The length of catalyst 90 is determined from both thecatalyst resonance time requirement and channel 87 flow rates. Tube 81serves as the outer channel barrier for the coolant channel 88, tube 81runs from the lower manifold (not shown) to an intermediate manifold 82.The intermediate manifold is a transition plate to allow the water flowchannel 88 from all the nested-flow units to enter into a common volume92 for water boiling, which is bounded by the larger circular tube 93.This common boiling volume permits the operation of the unit in either ahorizontal or vertical orientation; exit 94 is provided at the top ofthis volume based upon that orientation. The WSR nested-flow system issimilar in layout to the SMR nested-flow system, with the exception thatthe WSR nested-flow system results in all flows going through the unitwhile the SMR nested-flow system had all flows in the bayonetconfiguration. Assembled into a manifold nested-flow unit, the WSRproduction unit will be ten feet long and approximately twenty inches indiameter to meet the hydrogen production needs for twenty tons ofammonia per day. The efficiency improvement provided by the nested-flowtechnology will change the process energy needs from a process thatrequires external steam production at a cost of approximately$2.30/ton-of-ammonia to a process that is nearly self-sufficient ingenerating its own steam based on natural gas cost of $4/million-BTUs.

FIG. 7 is a cutaway view of a single nested-flow used for ureaproduction. Centerline 95 is included to remind the reader that thenested-flow is a nested tube assembly. Urea is produced by combiningcarbon-dioxide with ammonia at high pressure and slightly elevatedtemperatures. Tube 96 is the pressure boundary of a boiling volume usedto control the operating temperature of the urea formation process. Thepressure within volume 111 is regulated to control the boilingtemperature. Tube 96 is approximately two feet in diameter and enclosesall of the single nested-flow unit for urea production. For non-portableplants, this tube could be up to twenty feet long, while for portableplants, this tube may be as short as six feet long. Tube 97 isapproximately 3 to 4 inches in diameter and is the outer tube andpressure boundary of the single nested-flow unit. The nested-flow unitlength is dependent upon site constraints and could be as short as sixfeet or as long as twenty feet. Tube 97 is constructed of thick walledstainless steel for pressure requirements. “Wire” wrapped tube 98 andall tubes nested within tube 98 are non-metallic to providecompatibility with the ammonia-carbamate being formed in the down-flowchannel 112. “Wire” wrapped tube 99 provides an initial formationchannel for the ammonia-carbamate which is only slightly more dense thanwater. The ammonia-carbamate flows to the lower end of channel 112, andthen flows up between tube 99 and tube 100. Tube 100 is a tube with manycascading ledges to slow the fall of the urea/ammonia-carbamate mixtureas this mixture flow down the tube. O-ring grove 101 provides one methodof sealing the non-metallic nested tubes to the manifold plates.Manifold plate 102 provides the upper flow path for the boiling flow 117that cools all the single nested-flow units within this manifoldnested-flow unit. Plate 103 serves as the lower boundary for thatboiling flow and the upper boundary for one of the urea reactants flow,carbon-dioxide. Plate 104 serves as the lower boundary for that reactantflow and the upper plate of the remaining reactant flow, ammonia. Plate105 serves as the lower boundary for the second reactant flow and theupper plate of the urea discharge flow. Plate 106 serves as the lowerplate for the urea discharge flow. Flow 117 is the liquid flow into thecommon boiling region 111, while flow 108 is the gaseous discharge fromthat region. Flow 118 is the common flow of the reactant,carbon-dioxide. This reactant flow through nested-flow channel 113,providing a sweep gas to assure no ammonia-carbamate enters this channeland corrodes the pressure boundary provided by tube 97. At location 110,the carbon-dioxide gas enters the non-metallic flow channel 112 where itreacts with the ammonia reactant and forms ammonia-carbamate. Thisammonia-carbamate flows down channel 112 then up channel 114, providingsufficient resonance time to convert approximately 80% of theammonia-carbamate into urea. The urea/ammonia-carbamate mixture thenspills over tube 100 at the upper end 109 and cascades down a series oflandings to permit the ammonia reactant flow to strip the urea ofammonia-carbamate by subjecting the mixture to a constantly decreasingcarbon-dioxide partial pressure. This stripping action results inextremely low levels of ammonia-carbamate in the urea flow at the lowerend of tube 100. All single nested-flow units combine their urea flowinto manifold channel 120 where it is discharged from the productionunit. Flow channel 119 provides the common entrance point for ammonia.Ammonia reactant is used as a stripping gas to remove theammonia-carbamate from the down-flowing liquid mixture. Ammonia is usedas the stripping gas to suppress the formation of Biuret, a chemicalthat is damaging to some crops. At region 116, the carbon-dioxide vaporpressure is essentially zero, while the vapor pressure of the ammonia isessentially the operating pressure of the urea production system. Thisammonia flows counter to the urea/ammonia-carbamate mixture, the mixturebeing a liquid and the reactant being a gas. As the reactant flow movesup tube 100, the vapor pressure of the carbon-dioxide increases due tothe decomposition of the ammonia-carbamate as a direct result of thereduced carbon-dioxide partial pressure. Eventually the ammonia reactantflow 107 exits the top of tube 100 carrying with it all of thecarbon-dioxide and ammonia released from the decomposition ofammonia-carbamate from this liquid/gas counter-flow stripping reaction.All of this gas flow is then mixed with the feed carbon-dioxide reactantflow in channel 112 where the deficiency in the carbon-dioxide partialpressure is made up by the feed of carbon-dioxide reactant flow throughopening 110 and ammonia-carbamate once again is formed. Configured as 6ft tall columns, a total of 110 columns will be required to obtain aresonance time of approximately 20 minutes. One arrangement could bethree units each 24 inches in diameter and 80 inches tall. This ureaproduction nested-flow system provides a compact and economic system forthe ammonia-carbamate formation, urea formation and stripping step forremoving residual ammonia-carbamate form the urea.

Deployment of the Invention

These chemical reactors permit the modularization of a fertilizer plant.One modularization option would be to house these chemical reactors intostandard forty feet long eight feet wide and eight feet tall shippingcontainers. To meet production goals of thirty-five tons per day ofurea, only two shipping containers would be required to house all thechemical reactors, purification equipment, pumps and compressors. Suchan arrangement would permit the equipment to be deployed rapidly to bothregions that are short of fertilizer production capability and locationswhere natural gas is being wasted through flaring operations. Forlocations that are not supported or under-supported with electricalpower, hydrogen production can be increased to provide additionalhydrogen for use in hydrogen based fuel cells for production of neededelectrical power. To meet 20 tons/day ammonia or 35 tons/day ureaproduction, 1½ million cubic feet per day of hydrogen measured at STPwill be required. If the hydrogen production is doubled, a 35 ton/dayurea production plant could provide approximately 3.5 MW of electricalpower using hydrogen fuel cell technology.

What is claimed is:
 1. A process for making ammonia, the processcomprising: flowing a reactant comprising hydrogen and nitrogen in afirst flow path in a first direction, wherein: a first tube defines aportion of the first flow path, the first tube is characterized by afirst diameter, and the first tube has a first end; reacting thehydrogen and the nitrogen through the first flow path to produce aproduct comprising ammonia; flowing the product through a second flowpath in a second direction, the second direction being opposite thefirst direction, wherein: the first flow path is in fluid communicationwith the second flow path, the first flow path and the second flow pathare coaxial about a longitudinal axis, a second tube and the first tubedefine a portion of the second flow path, the second tube ischaracterized by a second diameter, the second diameter is greater thanthe first diameter, and the second tube has a second end; flowing theproduct through an outlet defined by a manifold assembly, wherein: theoutlet is in fluid communication with the second flow path, and themanifold assembly is in contact with the second end.
 2. The process ofclaim 1, wherein: reacting the hydrogen and the nitrogen comprises usinga catalyst, and the catalyst is disposed to define a portion of thesecond flow path.
 3. The process of claim 1, wherein the first flow pathand the second flow path are in a bayonet configuration.
 4. The processof claim 1, wherein: the reactant comprises one molar part nitrogen andthree molar parts hydrogen.
 5. The process of claim 1, wherein: theproduct comprises unreacted nitrogen and unreacted hydrogen.
 6. Theprocess of claim 1 wherein: the first flow path and the second flow pathare annular flow paths.
 7. The process of claim 1, wherein: a third tubeand the first tube define the portion of the first flow path, the thirdtube is characterized by a third diameter, the third diameter is lessthan the first diameter, and the third tube defines a cavity that is notin fluid communication with the first flow path.
 8. The process of claim7, further comprising: measuring a temperature using a thermocoupleinserted in the cavity.
 9. The process of claim 1, wherein the firstdiameter is from 0.375 inches to 0.875 inches.
 10. The process of claim1, wherein the second diameter is from 0.50 inches to 1.00 inches. 11.The process of claim 1, wherein: the second tube is characterized by alength, and the length is from 10 feet to 20 feet.
 12. The process ofclaim 1, further comprising: flowing a coolant through a third flowpath, transferring heat from the product to the coolant, wherein: thesecond tube defines a first portion of the third flow path.
 13. Theprocess of claim 12, wherein: a third tube defines a second portion ofthe third flow path, the third tube is characterized by a thirddiameter, and the third diameter is greater than the second diameter.14. The process of claim 1, wherein: the second end is the end of thesecond tube closest to the first end, the manifold assembly is incontact with the first end, the manifold assembly defines an input, andthe input is in fluid communication with the first flow path, theprocess further comprising: flowing the reactant through the input. 15.The process of claim 14, wherein: the second tube and a third tubedefine a portion of a third flow path, the third tube is characterizedby a third diameter, the third diameter is greater than the seconddiameter, the third tube has a third end and a fourth end, the third endis closest to the first end, and an exit port is at the fourth end,further comprising: flowing a coolant through the third flow path,transferring heat from the product to the coolant, and flowing thecoolant through the exit port.
 16. The process of claim 14, furthercomprising: condensing the ammonia in the second flow path to formliquid ammonia at the outlet.
 17. The process of claim 16, wherein: thelongitudinal axis is horizontal, a first port and a second port are influid communication with the outlet, the first port is disposed at aposition lower than the second port, the process further comprising:separating the liquid ammonia from unreacted hydrogen and unreactednitrogen using the first port and the second port.
 18. A process formaking ammonia, the process comprising: flowing a reactant comprisinghydrogen and nitrogen through an input; flowing the reactant in a firstflow path in a first direction; reacting the hydrogen and the nitrogenthrough the first flow path to produce a product comprising ammonia;flowing the product through a second flow path in a second direction,the second direction being opposite the first direction; and flowing theproduct through an outlet; wherein: a portion of the first flow path isdefined by a first tube, a portion of the second flow path is defined bythe first tube and a second tube, the first flow path and the secondflow path are annular flow paths, the first tube has a first end, thesecond tube has a second end, the second end is the end of the secondtube closest to the first end, a manifold assembly is in contact withthe first end and the second end, the manifold assembly defines theinput and the outlet, the input is in fluid communication with the firstflow path, and the outlet is in fluid communication with the second flowpath.
 19. A process for making ammonia, the process comprising: flowinga reactant comprising hydrogen and nitrogen in a first flow path in afirst direction, wherein: a first tube defines a portion of the firstflow path, and the first tube is characterized by a first diameter;reacting the hydrogen and the nitrogen through the first flow path toproduce a product comprising ammonia; flowing the product through asecond flow path in a second direction, the second direction beingopposite the first direction, wherein: the first flow path is in fluidcommunication with the second flow path, the first flow path and thesecond flow path are coaxial about a longitudinal axis, a second tubeand the first tube define a portion of the second flow path, the secondtube is characterized by a second diameter, the second diameter isgreater than the first diameter, a third tube and the first tube definethe portion of the first flow path, the third tube is characterized by athird diameter, the third diameter is less than the first diameter, andthe third tube defines a cavity that is not in fluid communication withthe first flow path.
 20. The process of claim 19, further comprising:measuring a temperature using a thermocouple inserted in the cavity.